1. Field of the Invention
The present invention relates to composite modules and, particularly, to a composite module, such as a DC-DC converter, which includes a multilayer circuit board that defines a main substrate and includes a principal surface on which functional elements, such as semiconductor devices and capacitors, are mounted.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2007-281160 describes a composite module which includes a multilayer circuit board that defines a base and includes a principal surface on which functional elements, such as a semiconductor device and a capacitor, are mounted and covered with a resin member. The resin member is provided with holes that are formed by a laser or other suitable method to extend to the multilayer circuit board. The holes are filled with electrode paste to form via-hole conductors, which are connected at one end to input/output terminals on a surface of the resin member. Thus, a compact composite module can be obtained in which functional elements are mounted on both principal surfaces of the multilayer circuit board.
This composite module enables a plurality of functional elements to be mounted on both principal surfaces of the multilayer circuit board. However, when the number of surface mount devices, such as capacitors and inductors, on the multilayer circuit board increases, a required number of via-hole conductors cannot be formed in the resin member without increasing the area of the multilayer circuit board. As a result, the size of the composite module itself is increased. Avoiding such a size increase would require a reduction in the number of via-hole conductors. However, reducing the number of via-hole conductors makes it difficult to enable a large number of components to be mounted on the multilayer circuit board. In other words, since it has been difficult to achieve both high-density mounting of components and size reduction, it has been necessary to sacrifice one of high-density mounting of components or size reduction.